The subject invention relates to feeding of single sheets of paper or the like from a stack of sheets for processing by folders, printers, copiers or the like. More particularly, it relates to detecting double fed sheets which occur when a sheet feeder fails to properly singulate sheets from the stack.
In printers, copiers, inserters, and similar such systems it is frequently necessary to singulate sheets from a stack of sheets for further processing by the system. In inserters, the singulation of envelopes is also known. Sheet feeding or envelope feeding apparatus for performing this singulation function are well known, and, in general, they are effective. However, inevitably such sheet feeders will fail to singulate and will feed a "double" sheet (i.e., two or more overlapping sheets). Such double fed sheets may result in sheets being grouped incorrectly for further processing. The double fed sheets may also jam in the system, requiring operator intervention to clear the jam. Perhaps more importantly, if the sheets contain information or are otherwise unique (e.g., return of cancelled checks) then their destruction in a jam caused by a double feed may significantly interfere with operations.
For these reasons it is known to provide such systems with detectors downstream from the sheet feeder to detect double fed sheets before a jam and possible destruction of the sheets can occur. One known method is to use an optical system to measure the transparency of a sheet after it is fed from the sheet feeder. Another known method uses precise, sensitive mechanical switches to detect an increase in the thickness of a fed sheet. Both of these methods for detecting double fed sheets involve precise, painstaking adjustments each time the type of sheet to be fed is changed.
Another known method uses a roller/arm assembly coupled to a distance measuring device such as a hall sensor. A primary object for such methods and apparatus of detection is to accurately and reliably detect "double" feeds without causing damage to the sheets being fed. Achieving this object is a problem when the feeder must handle sheet material of varying thicknesses. If a double detect apparatus is configured to detect thick sheets or envelopes, there is a likelihood that if thin sheets or envelopes are fed, detection of double feed will not occur. Conversely, if the apparatus were set to detect thin sheets or envelopes, then feeding thick sheets or envelopes may cause jamming or stalling of the feeding process.
Heretofore, any adjustment to the double detect apparatus, for example to handle sheets of varying thickness, has generally been performed by a service technician. Since sheet feeders can be operator adjusted to handle sheets of varying thicknesses, the double detect mechanism is a hindrance to an operator making such a change without a service call.
U.S. Pat. Nos. 4,879,513 and 4,378,109 disclose devices for measuring the thickness of paper on surface. However, the devices do not include adjustments means for handling thick or thin material.